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| United States Patent |
6,143,065
|
|
Freeman
, et al.
|
November 7, 2000
|
Precipitated calcium carbonate product having improved brightness and
method of preparing the same
Abstract
A method of significantly improving the brightness and shade properties of
a high quality PCC product. The method includes providing a calcium
containing feed source that contains discoloring impurities. The calcium
containing feed source is formed into an aqueous slurry of calcium
carbonate. Thereafter, the slurry is fine screened through a 325 mesh
screen in order to yield a slurry of filler grade calcium carbonate. The
filler grade calcium carbonate slurry is wet milled in order to liberate
the discoloring impurities and reduce the median particle size of the
calcium carbonate to less than 2 microns. The discoloring impurities are
then magnetically separated from the milled calcium carbonate slurry by
subjecting the slurry to a high intensity magnetic filed. After the
magnetic separation step, the purified slurry may be de-watered to yield a
dry powder; or alternatively, the slurry may be retained in aqueous form.
In any event, the resultant calcium carbonate product has a median
particle size of less than 2 microns and a TAPPI brightness of greater
than 96.
| Inventors:
|
Freeman; Gary Michael (Macon, GA);
Harrison; John Mecaslin MacGeoghegan (Twiggs, CO);
Lunden; Klaus A. (Kokkedal, DK)
|
| Assignee:
|
J. M. Huber Corporation (Edison, NJ)
|
| Appl. No.:
|
351473 |
| Filed:
|
July 12, 1999 |
| Current U.S. Class: |
106/464; 423/430; 423/432 |
| Intern'l Class: |
C09C 001/22 |
| Field of Search: |
106/464
423/430,432
|
References Cited
U.S. Patent Documents
| 3920800 | Nov., 1975 | Harris | 423/432.
|
| 3961971 | Jun., 1976 | Abercrombie, Jr. et al. | 106/72.
|
| 3980240 | Sep., 1976 | Nott | 241/20.
|
| 3984309 | Oct., 1976 | Allen et al. | 209/214.
|
| 4165840 | Aug., 1979 | Lewis et al. | 241/20.
|
| 5084254 | Jan., 1992 | Golley | 423/430.
|
| 5292365 | Mar., 1994 | Delfosse | 106/464.
|
| 5846500 | Dec., 1998 | Bunger et al. | 423/155.
|
| 5879442 | Mar., 1999 | Nishiguchi et al. | 106/464.
|
| 5939036 | Aug., 1999 | Porter et al. | 423/432.
|
Primary Examiner: Bell; Mark L.
Assistant Examiner: Faison; Veronica F.
Attorney, Agent or Firm: Nieves; Carlos
Claims
What is claimed is:
1. A precipitated calcium carbonate product comprising a plurality of
calcium carbonate particles having a median particle size of less than
about 2.0 .mu.m, a TAPPI Brightness of at least about 97, and a 75/25
slope value of less than 2.0.
2. The precipitated calcium carbonate product of claim 1 wherein said
calcium carbonate particles have a median particle size of less than about
1.0 .mu.m.
3. The precipitated calcium carbonate product of claim 1 wherein said
calcium carbonate particles have a TAPPI Brightness of at least about 98.
4. A method of producing a fine particulate precipitated calcium carbonate
having TAAPI brightness of at least 96 comprising the steps of:
providing calcium oxide, said calcium oxide including discoloring
impurities;
forming said calcium oxide into an aqueous slurry of calcium carbonate;
deagglomerating said calcium carbonate slurry to liberate said discoloring
impurities and reduce the median particle size of said calcium carbonate
to less than about 2.0 .mu.m; and
magnetically separating said discoloring impurities from said
deagglomerated calcium carbonate slurry by subjecting said slurry to a
high intensity magnetic field.
5. The method of claim 4 further including the step of fine screening said
calcium carbonate slurry through a screen of from about 500 to about 200
mesh.
6. The method of claim 4 further including the step of dispersing said
calcium carbonate slurry with a dispersant prior to said step of
deagglomeration.
7. The method of claim 6 wherein said dispersant is added in an amount
sufficient to reduce the viscosity of said calcium carbonate slurry to
less than about 100 cps.
8. The method of claim 4 wherein said step of magnetic separation is
effected by subjecting said deagglomerated calcium carbonate slurry to a
magnetic field having an average intensity of from about 5 kilogauss to
about 20 kilogauss.
9. The method of claim 4 wherein said step of magnetic separation is
effected by subjecting said deagglomerated calcium carbonate slurry to a
high intensity magnetic field for about 0.5 to about 5 minutes.
10. The method of claim 4 further including the step of de-watering said
calcium carbonate slurry after said step of magnetic separation to yield a
calcium carbonate product having a median particle size of less than about
2 .mu.m and a TAPPI brightness of at least about 96.
11. The method of claim 4 further including the steps of:
slaking said calcium oxide in order to obtain a slurry of Ca(OH).sub.2
prior to said step of forming said feed source into an aqueous slurry of
calcium carbonate;
coarse screening said slurry of Ca(OH).sub.2 through a screen of from about
325 to about 50 mesh, and
carbonating said Ca(OH).sub.2 slurry in a reactor in order to obtain a
slurry of precipitated calcium carbonate.
12. The method of claim 11 further including the step of fine screening
said Ca(OH).sub.2 slurry through a screen of from about 500 to about 200
mesh prior to said carbonating step.
13. A method of producing a fine particulate precipitated calcium carbonate
having TAAPI brightness of at least 96 comprising the steps of:
providing calcium oxide, said calcium oxide including discoloring
impurities;
forming said calcium oxide into an aqueous slurry of calcium carbonate;
magnetically separating said discoloring impurities from said aqueous
slurry of calcium carbonate by subjecting said slurry to a high intensity
magnetic field; and
deagglomerating said calcium carbonate to reduce the median particle size
of said calcium carbonate to less than about 2 .mu.m.
14. The method of claim 13 further comprising the step of fine screening
said calcium carbonate slurry through a screen of from about 500 to about
200 mesh.
15. The method of claim 13 further including the step of dispersing said
aqueous slurry with a dispersant prior to said step of magnetic
separation.
16. The method of claim 15 wherein said dispersant is added in an amount
sufficient to reduce the viscosity of said aqueous slurry of calcium
carbonate to less than about 100 cps.
17. The method of claim 13 wherein said step of magnetic separation is
effected by subjecting said aqueous slurry of calcium carbonate to a
magnetic field having an average intensity of from about 5 to about 20
kilogauss.
18. The method of claim 13 wherein said step of magnetic separation is
effected by subjecting said aqueous slurry of calcium carbonate to a high
intensity magnetic field for about 0.5 to about 5 minutes.
19. The method of claim 13 further including the step of de-watering said
calcium carbonate slurry after said step of deagglomeration to yield a
calcium carbonate product having a median particle size of about less than
about 2 .mu.m and a TAPPI brightness of at least about 96.
20. The method of claim 13 further including the steps of:
slaking said calcium oxide in order to obtain a slurry of Ca(OH).sub.2
prior to said step of forming said feed source into an aqueous slurry of
calcium carbonate;
coarse screening said slurry of Ca(OH).sub.2 through a screen of from about
325 to about 50 mesh, and
carbonating said Ca(OH).sub.2 in a reactor in order to obtain a slurry of
precipitated calcium carbonate.
21. The method of claim 20 further including the step of fine screening
said Ca(OH).sub.2 slurry through a screen of from about 500 to about 200
mesh prior to said carbonation step.
22. A method of producing a fine particulate precipitated calcium carbonate
having a TAPPI brightness of greater than 96 comprising the steps of:
slaking calcium oxide containing discoloring impurities in order to obtain
a slurry of Ca(OH).sub.2 ;
coarse screening said slurry of Ca(OH).sub.2 through a screen of from about
325 to about 50 mesh;
magnetically separating said discoloring impurities from said Ca(OH).sub.2
slurry by subjecting said slurry to a high intensity magnetic field;
carbonating said Ca(OH).sub.2 slurry in a reactor in order to obtain a
slurry of precipitated calcium carbonate; and
deagglomerating said precipitated calcium carbonate slurry to reduce the
median particle size to less than about 2 .mu.m.
23. A paper composition including the precipitated calcium carbonate
product of claim 1.
Description
TECHNICAL FIELD
This invention relates to a calcium carbonate product having improved
brightness and, more particularly, to a coating grade, precipitated
calcium carbonate product that has a median particle size of less than 2
microns (.mu.m) and a TAPPI Brightness of greater than 96. The invention
also relates to a process of brightening a precipitated calcium carbonate
product by removing dark colored impurities through the steps fine
screening and magnetic separation.
BACKGROUND OF THE INVENTION
Calcium carbonate, CaCO.sub.3, occurs naturally in the form of limestone,
marble, chalk and coral. Powdered calcium carbonate is produced by either
chemical methods or by the mechanical treatment of the natural materials.
The term precipitated calcium carbonate applies to the commercial types of
the compound produced chemically in a precipitation process. The
precipitated products are generally finer in particle size, have a more
uniform particle size distribution and a higher degree of chemical purity.
A wide variety of calcium carbonate particle sizes and particle shapes can
be chemically produced via the precipitation processes. Calcium carbonate
is commonly precipitated in the form of calcite, in which the crystals are
typically either rhombohedral, cubic or scalenohedral in shape, or in the
form of aragonite, which is aciculir. Vaterite is another precipitated
form of calcium carbonate known in the art that is metastable.
Precipitated calcium carbonate is an extremely versatile filler and
pigment that is utilized in a wide variety of manufactured products
including paper, paint, plastics, rubber, textiles and printing inks.
Precipitated calcium carbonate (PCC) is used on a large scale in paper
filling and coating applications. PCC is utilized to increase the opacity
and brightness of paper. In addition to the desirable opacifying and
brightening characteristics, PCC provides a high resistance to yellowing
and aging of paper. In many high grade coating applications, a fine
particle size calcium carbonate is required (median particle size <2
microns). It is typically desirable for the calcium carbonate to be as
bright as possible in these high grade coating applications. However, it
is difficult to remove the fine dark colored impurities that are
introduced by the initial burnt lime source, which is commonly utilized as
the raw material in the PCC precipitation process. Such impurities have a
negative impact on the brightness and shade properties of the resultant
PCC reaction products after processing. More particularly, wet media
milling is a common step in the processing of coating grade PCC. It has
been found that wet media milling precipitated calcium carbonate generally
results in significant loses in pigment brightness due to the grinding of
the dark colored impurities present therein. Chemically, a burnt lime is
principally CaO, but examples of the impurities commonly found in the
burnt lime source include pyrite (iron sulfide), magnesium iron oxides,
calcium iron oxides, calcium sulfide and crystalline silicas. As the
particle size of the dark particle impurities is reduced through grinding
their tinctorial color strength increases dramatically thereby resulting
in significant loses in overall product brightness. The loss of PCC
pigment brightness from grinding can be on the order of 1.5 to 2.5 points
depending on the initial burnt lime source and degree of grinding.
Heretofore, in order to obtain a high quality PCC product with an
acceptable brightness, the CaO starting material that is utilized must be
of a high quality, i.e. low levels of impurities. If a high quality burnt
lime source is not readily accessible, significant logistics costs, about
10%, are added to the cost of the PCC product. Accordingly, the resultant
PCC product is relatively expensive.
SUMMARY OF THE INVENTION
The present invention is designed to overcome the deficiencies discussed
above. It is an object of the invention to provide a high grade,
precipitated calcium carbonate product that has a median particle size of
less than 2 microns and a TAPPI brightness of greater than 96.
It is a further object of the present invention to provide a process for
making a high grade, precipitated calcium carbonate product through the
use of magnetic separation and fine screening.
It is yet another object of the invention to provide such a process that
allows a lower quality CaO feed material (e.g., burnt lime) to be used so
that the resultant PCC product can be made from lower cost and/or more
readily available sources.
In accordance with the present invention, there is provided a method of
significantly improving the brightness and shade properties of a high
quality PCC product. The method includes providing a calcium containing
feed source that contains discoloring impurities. The feed source is
either a hydrated lime slurry (produced from the slaking of CaO) or an
aqueous slurry of calcium carbonate product. In one mode of the invention,
the hydrated lime slurry is subjected to magnetic separation using a high
gradient magnetic field in order to remove the discoloring impurities.
After this purification, the hydrated lime slurry is then carbonated in a
reactor to yield a resultant PCC product of high brightness. Hydrated lime
slurries are also commonly referred to in the literature as milk of lime
(MOL). More commonly, however, the preferred calcium containing feed
source is a PCC reactor product delivered in a slurry form from a
carbonation reactor. Thereafter, the PCC slurry is fine screened through a
325 mesh screen (45 .mu.m) in order to yield a filler grade calcium
carbonate slurry. The filler grade calcium carbonate slurry is wet milled
in order to reduce the median particle size of the calcium carbonate to
less than 2 microns and thereby liberates the discoloring impurities that
were entrained therein. The discoloring impurities are then magnetically
separated from the deagglomerated calcium carbonate slurry by subjecting
the slurry to a high intensity magnetic field. After the magnetic
separation step, the purified slurry may be de-watered to yield a dry
powder; or alternatively, the PCC slurry may be retained in aqueous form
and concentrated as desired. In any event, the resultant calcium carbonate
product preferably has a median particle size of less than 2 microns and a
TAPPI brightness of greater than 96.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graphical representation showing the effect of screening milk
of lime (MOL) on the brightness of the resultant filler grade and coating
grade PCC products produced therefrom, and
FIG. 2 is a graphical representation showing the impact of increasing the
magnet residence time of the MOL on brightness of the resultant filler
grade and coating grade PCC produced therefrom.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
In accordance with the preceding summary, the present invention is directed
toward a method of obtaining high grade, precipitated calcium carbonate
products having a median particle size of less than 2 microns and a TAPPI
brightness of greater than 96, and more preferably, a particle size of
less than 1 micron and a TAPPI brightness of greater than 97. The
precipitated calcium carbonate products of the present invention are
obtained through a combination of magnetic separation and fine screening.
More specifically, the present inventive process includes providing a
calcium containing feed source for magnetic separation that contains
discoloring impurities. The calcium feed source used for making PCC
products is typically burnt lime (CaO). Burnt lime is also commonly
referred to as quick lime or calcined limestone. The discoloring
particulates in the burnt lime appear to principally consist of calcium
iron oxide that is likely produced during the high temperature calcination
of natural limestone as a result of the presence of pyritic impurities.
Other feed sources for calcination into burnt lime include: chalk, coral
and marble. The CaO feed source can be purified by magnetic separation
once it has been slaked into a milk of lime slurry. The purified milk of
lime slurry is then subsequently used to produce a high quality PCC
product of high brightness. Additionally, the initial calcium containing
feed source for magnetic separation can be a precipitated calcium
carbonate as more fully described below.
When the initial calcium containing feed source for magnetic separation is
a precipitated calcium carbonate, the entire inventive process includes
the step of adding burnt lime to a slaker where the CaO is hydrated to
yield milk of lime (also known as calcium hydroxide or Ca(OH).sub.2). The
milk of lime slurry is preferably hydrated to about 20% solids by weight,
however hydration can take place to varying ranges of percent solids. The
milk of lime is then coarse screened through about a 140 to about 50 mesh
screen, and more preferably through a 100 mesh screen. In the preferred
method, the Ca(OH).sub.2 is then fine screened through about a 200 mesh to
about 500 mesh screen and, even more preferably, through a 325 mesh screen
to remove sand and other dark particulate matter. It should be noted that
the step of fine screening can take place at other stages of the inventive
process as more fully described below. The % solids of the fine screened
slurry of Ca(OH).sub.2 is then preferably reduced to about 12 to 15% by
weight. Thereafter, the calcium hydroxide slurry is introduced into a PCC
reactor where it is carbonated to yield a slurry of precipitated calcium
carbonate. The product that comes out of the PCC reactor is referred to
herein as a filler grade precipitated calcium carbonate slurry. It should
be noted that the filler grade PCC slurry can be fine screened after it
comes out of the PCC reactor. This filler grade precipitated calcium
carbonate slurry, after dispersion with an anionic dispersant to reduce
its viscosity, can serve as a calcium feed source for magnetic separation.
The filler grade precipitated calcium carbonate slurry delivered by the PCC
reactor is typically from about 15 to about 20% solids. In a preferred
method, this PCC slurry is decanted to a filler grade PCC containing about
50% solids by weight. The decanting is preferably accomplished by means of
mechanical de-watering with a centrifuge. Alternatively, the PCC product
can be filtered by means of a filter press or similar filtering devices.
Thereafter, the filler grade PCC is placed into a high speed mixer of a
type known in the art and dispersed into a low viscosity slurry,
preferably with an anionic dispersant such as a sodium polyacrylate
(NaPA). The dispersant is added at this point to improve distribution of
the solids within the liquids and allow efficient, subsequent wet
grinding. The dispersant is added in an amount sufficient to reduce and
keep the Brookfield viscosity of the slurry to less than about 100 cps (at
20 rpm). It is noted that upon wet grinding, the slurry viscosity will
increase.
After the dispersant is added to the slurry and good distribution is
obtained, the mixture, which is preferably at about 50% solids by weight,
is then transferred to a wet grinding media mill. One preferred wet
grinding mill is a Drais Mill manufactured by Draiswerke, Inc., Mahway,
N.J. The Drais mill is a horizontal style media mill. To produce a PCC
coating product, the dispersed, filler grade PCC slurry is then wet milled
in order to deagglomerate the PCC into smaller aggregates or its
individual crystals. Alternatively, deagglomeration can be achieved by use
of a high shear, rotor-stator type mixer. The rhombohedral particle form
is the preferred particle form for PCC which is used in high grade coating
applications. It should be noted that the inventive process set forth
herein can also be applied to obtain other fine particle size, coating
grade PCC products of various morphologies and crystal structures.
It has been found that when the particle size of the milled PCC is reduced
to below 2 microns, as measured by a Sedigraph particle size analyzer
(Model 5100, manufactured by Micromeritics Instrument Corp., Norcross,
Ga.), the brightness drops to undesirable levels. The brightness drop
appears to occur as a result of the grinding of iron containing impurities
which takes place at the media milling stage. Brightness drops on the
order of 1.5 to 2.5 points (depending on the initial burnt lime source)
when the particle size is reduced to below 0.6 microns. To prevent the
drop off in brightness, the present inventors have discovered that by
subjecting the media milled PCC slurry to a high intensity magnetic field,
the "magnetic" dark particulates can be separated out and a high purity,
coating grade PCC slurry can be obtained. Factors affecting magnetic
separation include the intensity of the magnetic field, the fineness of
the steel wool matrix employed in the magnet's canister, the % solids of
the PCC slurry, the viscosity of the slurry and the residence time in the
magnetic separator.
Accordingly, the present inventive method includes the step of passing the
wet milled, PCC slurry through a wet, high intensity magnetic separator.
One known type of magnetic separator is a continuous flow magnetic
particle separator of the type described in U.S. Pat. No. 3,983,309 to
Allen et al., the contents of which are incorporated herein by reference.
A preferred magnetic separator is the High Gradient Magnetic Separator
(HGMS) available from Erie, Magnetics, Inc., Erie, Pa.; Pacific Electric
Motors, Inc. (PEM), Oakland, Calif.; Carpco, Jacksonville, Fla.; and
others). This high intensity magnetic separator is effective in separating
fine, submicron sized impurities of a paramagnetic nature as well as the
more strongly magnetic ferromagnetic particles.
As previously noted, the step of magnetic separation can take place at
other stages of the inventive process. For example, the discoloring
impurities can be magnetically removed from the calcium hydroxide slurry
prior to its carbonation into precipitated calcium carbonate. Further, the
magnetic separation can take place prior to or immediately following the
stage where the PCC filler grade slurry is fine screened. However, it has
been found that the final brightness benefits obtained are the highest
when the dispersed, milled PCC slurry is subjected to magnetic separation.
After the magnetic separation step, the purified slurry may be de-watered
to yield a dry powder; or alternatively, the slurry may be retained in
aqueous form and concentrated as desired. The de-watering step typically
is effected via an evaporator in a manner known in the art. The purified
slurry may also be fine screened at this stage. In any event, the
resultant PCC product preferably has a median particle size of about less
than 2 microns and a TAPPI brightness of at greater than 96. TAPPI
brightness method used herein is T646 om-86 "Brightness of clay and other
minerals." Brightness is measured utilizing a Technibrite Model TB-1C
brightness meter available from Technidyne Corporation, New Albany, Ind.
In order to demonstrate the efficacy of the present inventive process, a
number of illustrative Examples and Tables follow. Examples I and II show
the effect of magnetic separation on brightness and shade characteristics
of samples of rhombohedral PCC products prepared from Bedford and
Marbleton, Quebec, Canada burnt lime referred to herein as Bedford PCC or
Marbleton PCC, respectively. The CaO feed sources in Examples I and II
were PCC quality burnt limes having a Fe.sub.2 O.sub.3 content of less
than 0.2% and a MnO content of less than 0.007%. The iron and manganese
content present in the burnt lime feed is well known to have a direct
bearing on the resultant PCC brightness values obtained. The burnt lime
utilized in Examples I and II was hydrated in a slaker and then converted
into a filler grade PCC product in the manner described above. The
magnetic separator utilized in Examples I and II was a 2 Tesla Field
Strength, lab-scale magnet unit equipped with a 25-30 micron ultrafine
fiber steel wool matrix utilizing a 4 minute retention time with 4
canisters. The field intensity of the magnet was 20 kilogauss. The PCC
filler or coating products fed into the magnetic separator were of about
20% solids. To facilitate good magnetic separation, Accumer 9300 sodium
polyacrylate dispersant available from Rohm & Haas, Philadelphia, Pa. was
added to the "magnet feed" slurry to reduce the viscosity to <50 cps at 20
rpm.
EXAMPLE I
A 20% solids slurry of Bedford PCC (filler grade) was provided. The PCC
pigment had a TAPPI brightness of 97.23, a median particle size of 1.23
microns (Sedigraph) and a 75/25 slope value of 1.76.
The particle size "75/25 slope" values herein is a measure of a product's
particle size distribution. The lower the 75/25 slope value the more
narrow the particle size distribution. Conversely, the higher the 75/25
slope value the broader the particle size distribution. The particle 75/25
slope is measured as the ratio value of a pigment's particle size measured
in microns at the 75 percentile divided by the particle size measured in
microns at the 25 percentile. All particle size measurements were taken
with a Micromeritics Sedigraph 5100 X-ray sedimentation type instrument,
which uses Stokes Law in determining particle diameters. Hence, a PCC
coating pigment that has 75% of its particles <0.8 microns and 25% of its
particles <0.4 microns would therefore have a 75/25 slope value of
0.8/0.4=2.0.
The PCC filler slurry was then processed in one of the following ways:
1) The filler grade PCC slurry was screened to -325 mesh screen and to -500
mesh. The -325 mesh screened product had a TAPPI brightness of 97.25 and
the -500 mesh screened product had a TAPPI brightness of 97.38.
2) The filler grade slurry was wet ground in a Drais media mill to a median
particle size of 0.53 micron where it exhibited a TAPPI brightness of 95.6
and a slope of 1.66. The resulting PCC coating product was then screened
to -325 mesh where it exhibited a TAPPI brightness of 95.6 and to -500
mesh where it exhibited a TAPPI brightness of 95.62.
Alternatively, the wet milled PCC slurry was subjected to a step of
magnetic separation and the resulting product had a median particle size
of 0.52 microns and exhibited a TAPPI brightness of 97.83 and a slope of
1.67. Thereafter, the magnetically separated product was screened to -325
mesh where it exhibited a TAPPI brightness of 97.83 and to -500 mesh where
it exhibited a TAPPI brightness of 97.85.
3) The filler grade PCC slurry was magnetically separated and the resulting
product had a median particle size of 1.21 microns (Sedigraph) and
exhibited a brightness of 97.65 and a slope of 1.78. The magnetically
separated slurry was then screened to -325 mesh where it exhibited a TAPPI
brightness of 97.65 and to -500 mesh where it exhibited a TAPPI brightness
of 97.67.
Alternatively, the magnetically separated filler product was wet ground in
a Drais media mill to a particle size of 0.53 microns where it exhibited a
TAPPI brightness of 97.32 and a slope of 1.68. Thereafter, the
magnetically separated and milled product was screened to -325 mesh where
it exhibited a TAPPI Brightness of 97.30 and to -500 mesh where it
exhibited a TAPPI brightness of 97.34.
Standard US screens were employed. As used herein, "minus mesh" (-mesh)
means the material went through the screen and "plus mesh" (+mesh) means
material stayed on top of the screen. For example a product milled to -325
mesh goes through a 325 mesh screen and is therefor smaller than 325 mesh.
As can be seen in Example I, the best results, 2.2 point improvement, were
obtained by first wet grinding the filler grade PCC slurry, then
magnetically separating the discoloring impurities and finally fine
screening the resultant magnetically separated product. It is also noted
that when the filler grade PCC is subjected to magnetic separation prior
to wet grinding, brightness values decrease only 0.3 points upon grinding
as compared to a 1.6 point decrease upon grinding without prior magnetic
separation.
EXAMPLE II
A 20% solids slurry of Marbleton PCC (filler grade) was provided. The PCC
pigment had a TAPPI brightness of 97.23, a median particle size of 1.07
microns (Sedigraph) and a 75/25 slope value of 1.85. This slurry was then
processed in one of the following ways:
1) The filler grade PCC slurry was screened to -325 mesh and to -500 mesh.
The -325 mesh screened product had a TAPPI brightness of 97.24 and the
-500 mesh screened product had a TAPPI brightness of 97.28.
2) The filler grade slurry was wet ground in a Drais media mill to a median
particle size of 0.53 micron where it exhibited a TAPPI brightness of 95.
91 and a slope of 1.77. The resulting PCC coating product was then
screened to -325 mesh where it exhibited a TAPPI brightness of 95.90 and
to -500 mesh where it exhibited a TAPPI brightness of 95.92.
Alternatively, the wet milled PCC slurry was subjected to a step of
magnetic separation and the resulting product had a median particle size
of 0.54 microns and exhibited a TAPPI brightness of 97.95 and a slope of
1.68. Thereafter, the magnetically separated product was screened to -325
mesh where it exhibited a TAPPI brightness of 97.96 and to -500 mesh where
it exhibited a TAPPI brightness of 97.97.
3) The filler grade PCC slurry was magnetically separated and the resulting
product had a median particle size of 1.11 microns (Sedigraph) and
exhibited a brightness of 98.00 and a slope of 1.90. The magnetically
separated slurry was then screened to -325 mesh where it exhibited a TAPPI
brightness of 98.01 and to -500 mesh where it exhibited a TAPPI brightness
of 98.02.
Alternatively, the magnetically separated product was wet ground in a
Drais media mill to a particle size of 0.50 microns where it exhibited a
TAPPI brightness of 97.68 and a slope of 2.0. Thereafter, the magnetically
separated and milled product was screened to -325 mesh where it exhibited
a TAPPI Brightness of 97.67 and to -500 mesh where it exhibited a TAPPI
brightness of 97.69.
Once again, Example II demonstrated that the best results, a 2.0 point
improvement, were obtained by first wet grinding the filler grade PCC
slurry, then magnetically separating the discoloring impurities and
finally fine screening the resultant magnetically separated product.
EXAMPLE III
Two rhombohedral PCC coating products (derived from Bedford and Marbleton
limes, respectively) were produced in order to determine the effect of
field strength and residence time on the final brightness of the
magnetically separated PCC products. The PCC coating products in Example
III were produced at 20% solids by wet-grinding the PCC filler slurries in
non-dispersed form to a median particle size of 0.52 microns. Thereafter,
the products were each dispensed with Accumer 9300 NAPA and then subjected
to magnetic separation under varying magnet conditions to ascertain the
net effects on final product brightness. A PEM Magnet, Laboratory Model,
1"D.times.20" bore was utilized. As can be seen in Tables I and II,
retention times as low as 1 minute and magnet field strengths of from 5 to
20 kilogauss were explored.
TABLE I
______________________________________
Magnetic Field Strength Study of Bedford PCC
Sample ID
% Brightness
Whiteness
Yellowness
L a B
Feed 95.60 99.73 1.56 98.36 0.50 0.98
______________________________________
20 KG
4 Min RT
0-4 cans 97.86 94.90 0.89 99.20 0.50 0.52
4-8 cans 97.77 94.55 0.97 99.17 0.46 0.57
8-12 cans 97.59 94.13 1.05 99.12 0.41 0.63
2 Min RT
0-4 cans 97.71 94.55 0.95 99.14 0.48 0.56
1 Min RT
0-4 cans 97.73 94.49 0.98 99.17 0.49 0.58
16 KG
4 Min RT
0-4 cans 97.78 94.68 0.94 99.18 0.47 0.55
12 KG
4 Min RT
0-4 cans 97.68 94.63 0.92 99.12
0.51 0.54
8 KG
4 Min RT
0-4 cans 97.66 94.58 0.94 99.13 0.47 0.55
4 Min RT
0-4 cans 97.65 94.27 1.04 99.02 0.42 0.63
______________________________________
The number of canisters (referred to in the tables as "cans") refers to the
unit volume of material passing through the magnet and is related to cycle
time in the magnet. The more "cans", the more efficient the process. As
can be seen from Table I, the brightness of a composite sample of
canisters 8-12 was 97.59, dropping only 0.27 points from the 0-4 canister
composite.
TABLE II
______________________________________
Magnetic Field Strength Study of Marbleton PCC
Sample ID
% Brightness
Whiteness
Yellowness
L a b
Feed 95.91 91.34 1.46 98.49 0.51 0.91
______________________________________
20 KG
4 Min RT
0-4 cans 97.76 94.79 0.89 99.15 0.49 0.52
4-8 cans 97.73 94.55 0.97 99.17 0.43 0.57
8-12 cans 97.76 94.78 0.98 99.14 0.48 0.52
2 Min RT
0-4 cans 97.87 94.92 0.88 99.18 0.44 0.51
1 Min RT
0-4 cans 97.7 94.31 1.02 99.16 0.54 0.61
16 KG
4 Min RT
0-4 cans 97.85 94.9 0.89 99.2 0.44 0.52
12 KG
4 Min RT
0-4 cans 97.88 94.97 0.87 99.2 0.47 0.51
8 KG
4 Min RT
0-4 cans 97.75 94.59 0.94 99.15 0.46 0.56
5 KG
4 Min RT
0-4 cans 97.56 94.27 1.00 99.09 0.53 0.6
______________________________________
Tables I and II demonstrate that in processing low solids, PCC coating
slurries, in dispersed form, effective magnetic separation results as
based on TAPPI brightness response can be achieved with a 4 minute
retention time while utilizing magnet field strengths as low as 5 KG.
Additionally, when utilizing a magnetic separator at a field strength of
20 KG, a retention time as low as one minute was successfully utilized
with minimal negative affect on the resultant PCC brightness values. For
the Bedford based PCC coating product, TAPPI brightness decreased less
than 0.2 points when using retention times of 2 minutes and 1 minute,
while the corresponding brightness decrease for Marbleton based PCC
coating products was essentially zero.
EXAMPLE IV
A filler grade PCC slurry at 50% and 20% solids (produced from Bedford
lime) was provided to which Dispex 2695 dispersant available from Allied
Colloids, Suffolk, Va. was added on a 0.8% active weight basis. The
Bedford burnt lime used in making the filler grade PCC had the following
trace contaminant composition:
Fe.sub.2 O.sub.3 =0.137%
MnO=0.0041%
MgO=1.21%
SiO.sub.2 =0.697%
S=8290 ppm
A portion of the 50% or 20% solids, dispersed filler grade PCC slurry was
converted into a PCC coating control product by wet-grinding it in a
horizontal media mill to a target median particle size of 0.57 microns
(Sedigraph) and a 75/25 slope value of about 1.85. Portions of the PCC
filler product were magnetically separated at either 50% solids or 20%
solids using a Model A pilot magnetic separator operating at 16 KG using
various retention times and different numbers of canisters. Thereafter the
magnetically separated filler grade PCC products were wet-ground to
coating grade PCC with a target median particle size of 0.57 microns.
Brightness was measured on each of the paired filler and coating grade PCC
products to show the effect of different magnetic separation parameters on
coating grade PCC. The Model A pilot magnetic separator was equipped with
a 4.5 inch diameter by 18.5 inch long canister packed with a 25 micron
steel wool matrix. All magnetic separation runs were made at 16 KG using
retention times of 0.5 to 4 minutes. Tables III and IV demonstrate the
effects of magnetic separation on filler grade and coating grade PCC
slurries at 50% and 20% solids, respectively.
TABLE III
__________________________________________________________________________
Model "A" Magnet Experiments
Sedigraph
50% SOLIDS - Bedford PCC 75/25
ID Brightness %
White
Yellow
L a b MPS
Slope
__________________________________________________________________________
Filler (as rec)
97.46 93.75
1.12
99.07
0.51
0.68
0.85
1.79
Coating (control) 95.85 90.57 1.67 98.51 0.47 1.05 0.56 1.85
EXPT. #1
16 kG's 2 Min. R.T. 4 Canisters 50% Solids
Magnet 98.27 95.03
0.96
99.41
0.44
0.57
Wet-ground 98.10 94.81 0.99 99.37 0.43 0.59 0.57 1.81
16 kG's 2 Min R.T. 8 Canisters 50% Solids
Magnet 98.22 94.91
0.98
99.38
0.46
0.58
Wet-Ground 97.90 94.47 1.03 99.26 0.49 0.62 0.59 1.77
16 kG's 2 Min R.T. 16 canisters 50% Solids
Magnet 98.23 95.01
0.96
99.39
0.44
0.57
Wet-Ground 97.86 94.36 1.07 99.28 0.45 0.64 0.59 1.81
EXPT. #2
16 kG's 4 Min R.T. 4 Canisters 50% Solids
Magnet 98.57 95.68
0.84
99.51
0.48
0.49
Wet-Ground 98.39 95.53 0.85 99.44 0.46 0.49 0.59 1.77
16 kG's 4 Min R.T. 8 Canisters 50% Solids.
Magnet 98.38 95.42
0.86
99.42
0.49
0.50
Wet-Ground 98.05 94.85 0.95 99.30 0.46 0.56 0.59 1.77
16 kG's 4 Min R.T. 16 Canisters 50% Solids
Magnet 98.19 95.07
0.92
99.35
0.46
0.54
Wet-Ground 97.96 94.61 1.01 99.29 0.40 0.67 0.59 1.79
EXP. #3 16 kG's 1 Min R.T. 4 Canisters 50% Solids
Magnet 98.32 94.97
0.98
99.42
0.46
0.58
Wet-Ground 98.21 95.02 0.94 99.36 0.51 0.56 0.60 1.80
EXPT. #4
16 kG's 0.5 Min R.T. 4 Canisters 50% Solids
Magnet 98.39 95.19
0.93
99.44
0.50
0.55
Wet-Ground 98.17 94.83 0.99 99.36 0.49 0.59 0.61 1.76
__________________________________________________________________________
TABLE IV
__________________________________________________________________________
Model "A" Magnet Experiments
Sedigraph
20% SOLIDS - Bedford PCC 75/25
ID % Brightness
White
Yellow
L a b MPS
Slope
__________________________________________________________________________
Filler (as rec)
97.46 93.75
1.12
99.07
0.51
0.68
0.85
1.79
Coating (control) 95.85 90.57 1.67 98.51 0.47 1.05 0.56 1.85
EXPT. #5
16 kG's 2 Min. R.T. 4 Canisters 20% Solids
Magnet 98.64 95.74
0.84
99.54
0.42
0.49
Wet-Ground 98.53 95.65 0.81 99.42 0.42 0.47 0.56 1.88
16 kG's 2 Min R.T. 8 Canisters 20% Solids
Magnet 98.71 95.82
0.82
99.55
0.41
0.47
Wet-Ground 98.52 95.72 0.79 99.42 0.41 0.45 0.58 1.85
16 kG's 2 Min R.T. 16 Canisters 20% Solids
Magnet 98.58 95.43
0.91
99.51
0.39
0.53
Wet-Ground 98.53 95.72 0.80 99.44 0.36 0.46 0.58 1.88
EXPT. #6
16 kG's 4 Min R.T. 4 Canisters 20% Solids
Magnet 98.67 95.73
0.84
99.53
0.39
0.49
Wet-Ground 98.49 95.55 0.84 99.44 0.39 0.49 0.58 1.83
16 kG's 4 Min R.T. 8 Canisters 20% Solids
Magnet 98.57 95.52
0.88
99.50
0.41
0.52
Wet-Ground 98.46 95.49 0.85 99.43 0.40 0.50 0.58 1.9
16 kG's 4 Min R.T. 16 Canisters 20% Solids
Magnet 98.62 95.96
0.78
99.52
0.41
0.45
Wet-Ground 98.45 95.70 0.81 99.45 0.40 0.47 0.58 1.88
EXPT. #7
16 kG's 1 Min R.T. 4 Canisters 20% Solids
Magnet 98.63 95.80
0.82
99.54
0.40
0.48
Wet-Ground 98.48 95.82 0.78 99.45 0.40 0.45 0.59 1.90
EXPT. #8
16 kG's 0.5 Min R.T. 4 Canisters 20% Solids
Magnet 98.56 95.89
0.77
99.47
0.40
0.44
Wet-Ground 98.51 95.54 0.87 99.50 0.45 0.51 0.58 1.88
__________________________________________________________________________
As can be seen in Tables III and IV, better test results as reflected by
TAPPI brightness response were obtained when the slurry was diluted back
to 20% solids for processing through the magnet versus processing
conducted at 50% solids. The filler grade PCC subjected to magnetic
separation then wet-ground lost only 0.11 to 0.37 points brightness when
treated at 50% solids and 0.05 to 0.19 points when treated at 20% solids
products (rows labeled magnet minus, rows labeled wet-ground). This is
compared to 1.61 points brightness loss for the control pair which was not
magnetically separated. The coating grade PCC products of the invention
(rows labeled wet-ground) improved 2.0 to 2.7 points brightness as
compared to the coating control.
EXAMPLE V
A standard Bedford burnt lime was provided having the following impurities:
Fe.sub.2 O.sub.3 =0.177%
MnO=0.0053%
MgO=1.26%
SiO.sub.2 =0.738%
S=7050 ppm
The Bedford burnt lime was slaked at a 4:1 weight ratio of water/lime at a
slaking temperature of about 65.degree. C. After slaking was complete, the
milk of lime (MOL) slurry was either screened at a given mesh size prior
to carbonation reaction or screened through a 100 mesh screen and
subjected to magnetic separation prior to the carbonation reaction. In the
MOL screening experiments, screens of 60, 100, 200, 250 and 325 mesh were
respectively employed to remove the coarse particle impurities. For
magnetic separation experiments, a 1" High Gradient Magnetic Separator was
used and operated at 20 KG using retention times of 1, 2 and 4 minutes.
The screened or magnetically separated MOL feeds were then reacted in a
PCC reactor under typical carbonation conditions to produce a rhombohedral
PCC filler product having a BET surface area of about 7.5 m.sup.2 /g.
Citric acid was added to these MOL feeds (at 3 kg/metric ton) to inhibit
the formation of colloidal PCC during reaction. The PCC reaction products
were then post reactor screened to -325 mesh and subjected to low solids,
non-dispersed wet grinding to yield coating PCC product of about 0.6
micron MPS.
Table V demonstrates the effect of screen size on MOL used to make filler
grade PCC and coating grade PCC. FIG.1 is a graphical representation
showing the effect screen size has on the brightness of both filler grade
and coating grade PCC. Table VI demonstrates the effect of various magnet
retention times on 100 mesh screened MOL used to make filler grade and
coating grade PCC. FIG. 2 is a graphical representation showing the effect
of magnet residence time on filler grade PCC and coating grade PCC
prepared from magnetically separated, -100 mesh MOL.
TABLE V
__________________________________________________________________________
Physical Properties of PCC Prepared From Screened Bedford Lime
Sample
Sample
Lime MPS
75/25
ID Desc. Screens Brightness Whiteness Yellowness L a b .mu.m slope
__________________________________________________________________________
Filler Control
60 mesh
97.69
93.97
1.08 99.11
0.33
0.66
1.80
1.67
(Screen
Control)
Coating Control 60 mesh 96.61 93.13 1.03 98.56 0.49 0.62 0.57 2.10
(Screen
Control)
1A Filler
100 mesh
97.61
93.95
1.07 99.06
0.40
0.64
1.76
1.64
1B Coating 100 mesh 96.69 93.04 1.09 98.64 0.51 0.66 0.60 2.18
2A Filler 200 mesh 97.63 93.71 1.16 99.13 0.38 0.71 1.65 1.73
2B Coating 200 mesh 97.45 93.87 1.05 99.00 0.40 0.63 0.60 2.29
3A Filler 250 mesh 97.58 94.00 1.05 99.06 0.39 0.63 1.61 1.70
3B Coating 250 mesh 97.14 93.71 1.00 98.79 0.47 0.59 0.60 2.26
4A Filler 325 mesh 97.62 93.81 1.13 99.12 0.40 0.69 1.67 1.66
4B Coating 325 mesh 97.39 93.87 1.04 98.97 0.48 0.63 0.60 2.29
__________________________________________________________________________
TABLE VI
__________________________________________________________________________
Physical Properties of PCC Prepared From Magnetic Separated Bedford Lime
Sample
Sample
Lime MPS
75/25
ID Desc. Processing Brightness Whiteness Yellowness L a b .mu.m
__________________________________________________________________________
slope
Filler Control
325 mesh
97.52
93.62
1.17 99.11
0.45
0.72
1.73
1.7
(Magnet
Control)
Coating Control 325 mesh 96.66 93.58 0.92 98.56 0.50 0.54 0.59 2.26
(Magnet
Control)
5A Filler
325 mesh
97.75
93.63
1.24 99.25
0.42
0.76
1.75
1.71
(20 KG,
4 min RT)
5B Coating 325 mesh 97.52 94.12 1.02 99.07 0.46 0.61 0.61 2.28
(20 KG,
4 min RT)
6A Filler 325 mesh 97.77 93.71 1.22 99.26 0.40 0.75 1.72 1.76
(20 KG,
2 min RT)
6B Coating 325 mesh 96.75 93.61 0.93 98.61 0.41 0.55 0.62 2.2
(20 KG,
2 min RT)
7A Filler 325 mesh 97.80 93.87 1.18 99.26 0.41 0.73 1.66 1.77
(20 KG,
1 min RT)
7B Coating 325 mesh 96.98 93.76 0.93 98.68 0.39 0.55 0.61 2.2
(20 KG,
1 min RT)
__________________________________________________________________________
Although magnetic separation of the MOL feed yielded coating grade PCC
brightness improvements, the magnitude of the brightness benefit was
noticeably less than that derived when the step of magnetic separation was
effected on the wet milled PCC slurry. More specifically, at the highest
retention time of 4 minutes at 20 KG, TAPPI brightness was only improved
about 0.8 to 0.9 points.
It has been found that the present invention is an economic alternative to
securing high quality lime sources. The inventive process adds less than
2% to processing costs of coating grade PCC, while shipping costs for high
quality a lime source can typically add 10%.
The present invention may be embodied in other specific forms without
departing from the spirit or essential attributes thereof and accordingly
reference should be made to the appended claims rather than the foregoing
specification as indicating the scope of the invention.
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